The invention is directed to a broadband phase shifter circuit.
More specifically, the invention is directed to a broadband phase shifter circuit, particularly for an IQ modulator, comprising two phase shifter branches connected parallel to which the input signal to be shifted in phase is supplied at the input side and that supply phase-shifted output signals at their outputs shifted by a predetermined angle, such as 90.degree..
Broadband phase shifter circuits of this species are known; for example, they are employed as 90.degree. phase shifters for IQ modulators (for example, according to U. S. Pat. Nos. 4,908,532, 4, 951,000 or 5,644,260). However, these respectively have various disadvantages. The fashioning of the phase shifters as RC elements with variable capacitance diodes and their regulation via a phase detector connected between the outputs (U.S. Pat. No. 4,908,532) results in indefinite amplitudes of the output voltages at the two phase shifter branches. The different amplitudes deteriorate the modulation precision of the following IQ modulator. The opposed phase correction of two identical phase shifters in the two phase shifter branches via a phase detector (U.S. Pat. No. 4,951,000) is limited to specific phase shifters and cannot be employed, for example, given simple RC elements. The regulation of the two phase shifter branches dependent on the difference of the amplitude of the two output voltages (U.S. Pat No. 5,644,260) is not exact enough with respect to adhering to the desired, exact phase relationship.